Field of the Invention
The present invention relates to orthopedic cutting tools and, more particularly, to acetabular reamers having a domed shape for reaming a hemispherical cavity in the acetabulum.
Description of Prior Art
An acetabular reamer is a milling tool used in total hip arthroplasty (THA) to enlarge and reshape the acetabulum of the patient prior to implantation of a hip prosthesis. It typically consists of a substantially hemispherical steel cup having a plurality of cutter-hole combinations on its outer surface. The reamer is mounted axially onto the shaft of a power drill and manually pressed onto the implantation site. During rotation the cutters bite into and grate away the bone tissue of the ilium as the reamer is advanced into the acetabulum. The grated bone shavings and fragments are directed to the inner space of the reamer cup—called the debris chamber—via an ad hoc hole in front of each cutter. The accumulated bone shavings and fragments may subsequently be packed around the implanted prosthetic components to promote bone ingrowth for ensuring a tight fit between the bone and the prosthesis.
Prior art manufacturing processes for commercial, hemispherically shaped acetabular reamers are remarkably similar, usually involving some or all of following steps:                i. A thin-walled steel bowl-shaped cup blank is first fabricated by deep drawing, pressing or any equivalent method.        ii. In some cases, e.g. Salyer, U.S. Pat. No. 5,709,688, a sacrificial peripheral lip is incorporated in the design of the bowl-shaped cup blank to strengthen same and/or facilitate its handling during fabrication. When such a sacrificial peripheral lip is used, it is removed in a secondary operation before the acetabular reamer cup is completed.        iii. After deep drawing or pressing, the cup blank is perforated with a plurality of holes by punching, drilling or the like.        iv. If the holes are formed by a method that produces burs, the holes must be deburred.        v. Next, the edge of each hole is upset, countersunk or otherwise deformed outwardly to generate the cutting portion of the cutter.        vi. The cutting portion is then sharpened by sanding, grinding or the like, to yield a sharpened bevel and a cutting edge having the requisite sharpness for cutting bone.        vii. Finally, a back plate with crossbars or other system for connecting the reamer to the driver shaft is welded onto the reamer cup.        
Since the cup blank is a three dimensional, substantially hemispherical shape, producing the cutters using conventional machine tools is extremely complex and difficult. For this reason, the operations of stamping, countersinking, pressing open the edges of the holes as well as sharpening the raised cutting edges are generally carried out by hand. These processes are very slow and dependent upon the skill of the individual workman. Consequently, the quality or prior art reamers is typical of manual labor.
One issue with prior art reamers is the need for the surgeon to have at his disposal a wide range of acetabular reamers in different sizes from which he can choose the most appropriate ones for a specific surgical procedure. Consequently, commercial reamers are usually supplied in sets of different sizes. One reamer manufacturer, Tecomet, Inc., Wilmington, Mass., offers sets of 45 reamers in diameters ranging from 36 to 80 mm with the number of cutters per reamer varying between 26 and 68 per reamer. Considering that each cutter-hole combination may require perforation, deburring, upsetting of the holes and sharpening of the cutter edges, it can be seen that the average number of manual operations per reamer may exceed one hundred, making the fabrication of prior art acetabular reamers labor intensive and costly.
In addition to the labor intensive manufacturing processes, there are a number of functional issues with prior art reamers.
It is highly desirable that the bone be cut into a cavity that conforms as closely as possible in shape and dimensions to the acetabular cup being implanted to avoid implant instability. Therefore, those skilled in the art have sought to provide reamers that cut a cavity that is as close as possible to a precise hemispherical shape.
It is further desirable to advance the tool linearly into the bone to fully cut the cavity, versus having to rock the tool sideways by changing its orientation to achieve a complete shape. Most commercial reamers do not incorporate cutters in the equatorial region and therefore are unable to cut a full hemispherical cavity in the acetabulum without rocking. However, this may result in overcutting of the cavity. As a result, on some commercial reamers the hemispherical shape is extended with a cylindrical skirt or rim to accommodate equatorial cutters.
Another issue with prior art reamers is the cutting speed. The hemispherical reamer rotates on its rotational axis and thus a cutter's rotational speed is an inverse function of its latitude. This results in a dilemma. First, since the cutters near the apex or pole rotate much more slowly than those near the equator, surgeons have experienced a need to apply more strength to force the reamer against the bone in the final stages of reaming. Second, the higher rotational speed of equatorial cutters may result in higher friction and therefore generate elevated temperatures. Temperatures above 47° C. cause thermal osteonecrosis which contributes to implant loosening.
Yet another issue with prior art reamers is cutter path overlap. During reaming, each cutter tends to cut a latitudinal groove in the acetabulum. As a result, to produce a smooth, substantially hemispherical cavity in the acetabulum, it is desirable for the grooves of successive cutters to overlap longitudinally. For example, in Slone, Pub. 2013/0204254, cutting blades may be positioned on the acetabular reamer so as to entirely, in other words 100% overlap, while in Salyer, U.S. Pat. No. 5,709,688, the overlap between adjacent cutting edges is about one and one half times, or 100-150%. Clearly, the higher the overlap between paths of preceding cutters, the less work will have to be expended by the cutter, thus resulting in less torque and friction. This need for overlapping is all the more important in the apex or polar region, given the slower speed in that area.
In view of the foregoing, it would be desirable if the surgeon, by selecting the correct milling cutter, could be sure that the implant seat to be milled in the bone would accommodate the acetabular cup with as close a form fit as possible and with minimal expenditure of time and energy.
It would also be desirable to provide an improved acetabular reamer having cutters that are mechanically stronger and more resistant to abrasion so that their sharpness could be maintained over an extended period of use.
It is also highly desirable to provide an improved acetabular reamer which cuts faster and requires less force to be applied against the bone.
It is also desirable to provide an improved acetabular reamer which reduces the risk of thermal osteonecrosis by being produced from a material with higher thermal conductivity and/or having a lower coefficient of friction.
It is especially desirable to provide a reaming tool which can be produced more efficiently and economically by reducing or eliminating machining operations.
Finally, it is highly desirable to provide an improved acetabular reamer and method for making same which possess all of the above desirable features.
The instant invention meets all these needs.